An accurate determination of exposure to dental amalgam is essential in epidemiologic studies investigating the health effects of exposure to mercury from dental amalgams. The only available information regarding amalgam exposure from major health surveys is an indirect one derived from the FS component of the DFS scores. In the NIDR/USAF Amalgam Study surface-specific evaluations of all restorative materials were made and coronal caries were scored using the NIDR diagnostic criteria. Blood and urine mercury assays were also performed. The associations between exposure to dental amalgams and blood and urinary mercury concentrations were investigated for both the direct and indirect method (FS scores) of dental amalgam exposure. Six measures of amalgam exposure were investigated. Three direct estimates included: (1) the true total number of surfaces having an amalgam filling; (2) the number of posterior surfaces having an amalgam filling; and (3) the number of occlusal surfaces with an amalgam filling. The three indirect estimates were: (4) total number of surfaces having a filling; (5) total number of posterior surfaces with a filling; and (6) total number of occlusal surfaces with a filling. These male participants were 52.7 years old, ranging from 41 to 78 years of age. 4.9 percent of the participants were edentulous. The dentate participants had, on average, 23.5 natural teeth; a DFS score of 36.9; a total FS Score of 35.9; and a FS score of 10.2 on occlusal surfaces. They averaged 19.9 total surfaces with amalgams, 7.7 of which were on occlusal surfaces. 3.6 percent of the dentate group had no occlusal surfaces with amalgams, and 2.0 percent were amalgam free. The mean inorganic Hg concentrations in urine and whole blood were 2.9 ug/l and 0.6 ug/l, respectively. Indirect estimates of amalgam exposure produced positive biases ranging from 154 percent to 261 percent, depending on the age of the participant. The correlation between direct amalgam counts and FS scores was 0.65. Significant correlations between direct amalgam exposure and urinary inorganic Hg concentrations (0.34 and 0.31 for all surfaces and occlusals) were identified. Correlations were underestimated (0.16 and 0.23) by the indirect FS based estimate. Substantial misclassification for amalgam exposure level occurred by using the indirect FS based estimates.